Re: [Qemu-devel] [very-WIP 1/7] migration: Add VMSTATE_WITH_TMP

[Jianjun Duan <duanj@linux.vnet.ibm.com>]

On 10/16/2016 08:31 PM, David Gibson wrote:
> On Tue, Oct 11, 2016 at 06:18:30PM +0100, Dr. David Alan Gilbert (git) wrote:
>> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>>
>> VMSTATE_WITH_TMP is for handling structures where some calculation
>> or rearrangement of the data needs to be performed before the data
>> hits the wire.
>> For example,  where the value on the wire is an offset from a
>> non-migrated base, but the data in the structure is the actual pointer.
>>
>> To use it, a temporary type is created and a vmsd used on that type.
>> The first element of the type must be 'parent' a pointer back to the
>> type of the main structure.  VMSTATE_WITH_TMP takes care of allocating
>> and freeing the temporary before running the child vmsd.
>>
>> The post_load/pre_save on the child vmsd can copy things from the parent
>> to the temporary using the parent pointer and do any other calculations
>> needed; it can then use normal VMSD entries to do the actual data
>> storage without having to fiddle around with qemu_get_*/qemu_put_*
>>
If customized put/get can do transformation and dumping/loading data
to/from the parent structure, you don't have to go through
pre_save/post_load, and may get rid of parent pointer.

Thanks,
Jianjun

>> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> The requirement for the parent pointer is a little clunky, but I don't
> quickly see a better way, and it is compile-time verified.  As noted
> elsewhere I think this is a really useful approach which could allow a
> bunch of internal state cleanups while preserving migration.
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
>> ---
>>  include/migration/vmstate.h | 20 ++++++++++++++++++++
>>  migration/vmstate.c         | 38 ++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 58 insertions(+)
>>
>> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
>> index 9500da1..efb0e90 100644
>> --- a/include/migration/vmstate.h
>> +++ b/include/migration/vmstate.h
>> @@ -259,6 +259,7 @@ extern const VMStateInfo vmstate_info_cpudouble;
>>  extern const VMStateInfo vmstate_info_timer;
>>  extern const VMStateInfo vmstate_info_buffer;
>>  extern const VMStateInfo vmstate_info_unused_buffer;
>> +extern const VMStateInfo vmstate_info_tmp;
>>  extern const VMStateInfo vmstate_info_bitmap;
>>  extern const VMStateInfo vmstate_info_qtailq;
>>  
>> @@ -651,6 +652,25 @@ extern const VMStateInfo vmstate_info_qtailq;
>>      .offset     = offsetof(_state, _field),                          \
>>  }
>>  
>> +/* Allocate a temporary of type 'tmp_type', set tmp->parent to _state
>> + * and execute the vmsd on the temporary.  Note that we're working with
>> + * the whole of _state here, not a field within it.
>> + * We compile time check that:
>> + *    That _tmp_type contains a 'parent' member that's a pointer to the
>> + *        '_state' type
>> + *    That the pointer is right at the start of _tmp_type.
>> + */
>> +#define VMSTATE_WITH_TMP(_state, _tmp_type, _vmsd) {                 \
>> +    .name         = "tmp",                                           \
>> +    .size         = sizeof(_tmp_type) +                              \
>> +                    QEMU_BUILD_BUG_EXPR(offsetof(_tmp_type, parent) != 0) + \
>> +                    type_check_pointer(_state,                       \
>> +                        typeof_field(_tmp_type, parent)),            \
>> +    .vmsd         = &(_vmsd),                                        \
>> +    .info         = &vmstate_info_tmp,                               \
>> +    .flags        = VMS_LINKED,                                      \
>> +}
>> +
>>  #define VMSTATE_UNUSED_BUFFER(_test, _version, _size) {              \
>>      .name         = "unused",                                        \
>>      .field_exists = (_test),                                         \
>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>> index 2157997..f2563c5 100644
>> --- a/migration/vmstate.c
>> +++ b/migration/vmstate.c
>> @@ -925,6 +925,44 @@ const VMStateInfo vmstate_info_unused_buffer = {
>>      .put  = put_unused_buffer,
>>  };
>>  
>> +/* vmstate_info_tmp, see VMSTATE_WITH_TMP, the idea is that we allocate
>> + * a temporary buffer and the pre_load/pre_save methods in the child vmsd
>> + * copy stuff from the parent into the child and do calculations to fill
>> + * in fields that don't really exist in the parent but need to be in the
>> + * stream.
>> + */
>> +static int get_tmp(QEMUFile *f, void *pv, size_t size, VMStateField *field)
>> +{
>> +    int ret;
>> +    const VMStateDescription *vmsd = field->vmsd;
>> +    int version_id = field->version_id;
>> +    void *tmp = g_malloc(size);
>> +
>> +    /* Writes the parent field which is at the start of the tmp */
>> +    *(void **)tmp = pv;
>> +    ret = vmstate_load_state(f, vmsd, tmp, version_id);
>> +    g_free(tmp);
>> +    return ret;
>> +}
>> +
>> +static void put_tmp(QEMUFile *f, void *pv, size_t size, VMStateField *field,
>> +                    QJSON *vmdesc)
>> +{
>> +    const VMStateDescription *vmsd = field->vmsd;
>> +    void *tmp = g_malloc(size);
>> +
>> +    /* Writes the parent field which is at the start of the tmp */
>> +    *(void **)tmp = pv;
>> +    vmstate_save_state(f, vmsd, tmp, vmdesc);
>> +    g_free(tmp);
>> +}
>> +
>> +const VMStateInfo vmstate_info_tmp = {
>> +    .name = "tmp",
>> +    .get = get_tmp,
>> +    .put = put_tmp,
>> +};
>> +
>>  /* bitmaps (as defined by bitmap.h). Note that size here is the size
>>   * of the bitmap in bits. The on-the-wire format of a bitmap is 64
>>   * bit words with the bits in big endian order. The in-memory format
>